The present invention relates to a thermal line printer of the type having a line head composed of a plurality of linear thermal head segments aligned in the widthwise direction of a recording medium sheet perpendicular to the feeding direction or the lengthwise direction of the recording medium sheet so as to cover the entire span or width of the recording medium sheet.
One type of conventional divided line head is composed of a plurality of linear thermal head segments aligned in a single row and coupled to each other at opposed ends of adjacent segments. Each linear segment has a given length sufficient to cover the span of A4 or B4 size recording paper and is formed with a plurality of heating elements arranged linearly on the segment at a given pitch. These linear head segments are connected in series to each other to constitute the divided line head which can cover the entire span of larger size recording paper such as A1 size and A0 size, and which has a higher yield rate than that of a monolithic line head of comparative length.
However, this type of conventional divided line head has the drawback that the pitch of heating elements is made irregular at the junction or connecting portion of adjacent segments to thereby degradate the quality of printed image pattern.
Another type of conventional divided line head is disclosed in U.S. Pat. No. 4,660,052. This conventional head is composed of a plurality of linear thermal head segments aligned in a pair of parallel rows in staggered relation and in partially overlapping relation at end portions of the linear segments between the parallel rows so as to completely cover the entire width or span of recording paper. In operation, the first row of linear segments is activated to effect a part of single line printing, and then the second row of linear segments is shifted in the lengthwise direction of the recording paper relative thereto through an interval corresponding to the distance between the parallel rows and is activated to effect the remaining part of the single line printing to thereby complete the single line printing.
FIG. 1 is a side view of the conventional staggered thermal printing head device. As shown in the figure, the first row of head segments 1a and 1c are opposed to a corresponding first platen 2A, and the second row of head segments 1b and 1d are opposed to a corresponding second platen 2b. The major surfaces of the planar head segments 1a, 1b, 1c and 1d in contact with a recording paper 3 are aligned in parallel to a common plane tangential to the pair of platens 2a and 2b. An edge portion of the head segments may block a feeding pass of the recording paper transferred along the common tangential plane between the head segments and platen due to slight difference between the levels of the head segments and due to error in set angles of the head segments relative to the common tangential plane.
FIG. 2 is a partial side view of a thermal printing head segment. The head segment 1a is formed at is edge portion with electrically resistive heating elements 6 as well as common electrodes 4 for supplying driving current to the heating elements and a protective film layer 5, these of which are raised upwardly relative to the heating elements 6. The raised or convex portion tends to protrude into the feeding pass of the recording paper to thereby disturb the linearity of pass between the pair of the parallel platens 2a and 2b. Therefore, a dot line part printed by the downstream or succeeding thermal head segments 1a and 1c is not aligned or matched with the remaining dot line part printed by the upstream or preceding thermal head segments 1b and 1d. As a result, the conventional staggered printing head device cannot print a perfect dot line because of disturbance or distortion of recording paper pass.